The present invention relates to an image capturing apparatus incorporating a zoom lens system which performs variable power operations and focus adjustment by means of a driving action of a driving member.
Recently, the development of digital cameras has progressed remarkably. Improvements for achieving high pixel density up to mega-pixels, miniaturization equivalent to a silver-halide camera, cost reduction, etc. have progressed day by day. In addition, the number of digital cameras which incorporate an optical zooming system have been increased.
In the abovementioned trends, it has been considered to realize a low cost digital camera with a simple structure by means of a simplified zooming operation. A technique for this goal is set forth in Japanese Patent Tokkaihei 9-218336. The disclosed technique is intended to simplify the structure of the image capturing apparatus by employing a configuration in which a plurality of focal lengths are selected in a stepping mode, by changing focal lengths step by step, or by changing focal lengths in two or three stages by means of a conversion lens. Further, the disclosed configuration makes it possible to select a larger number of focal lengths by means of the electronic enlarging processing which enables the selection of an intermediate focal length.
Incidentally, although it is possible to select focal lengths in a stepping mode, the abovementioned technique does not include a focusing mechanism. Accordingly, the focus point is adjusted at the longest focal length at which the subject image is focused on CCD (Charge Coupled Device), and in the shorter focal length, the subject image can be focused on CCD as it is, since the depth of focus becomes deeper.
Employing a zoom lens having no focusing mechanism, however, the depth of focus becomes shallow when the zoom lens is set at the longer focal length, and the image quality of the subject in near distance is deteriorated when pan-focusing, in which subjects included up to infinity are captured within its depth of focus, is performed. Accordingly, in order to obtain high quality images, a focusing mechanism is required in addition to the zooming mechanism. A focusing mechanism, however, requires a separate driving mechanism including a motor and a control circuit, resulting in an increase of parts and production of a large-sized, high price image capturing apparatus.
Further, a large-sized apparatus results in an increases in weight and the driving mechanism of the lens rapidly drains batteries. As a result, in a worst case, an image capturing can not be reliably conducted by the user, due to the rapid dissipation of the battery.
On the other hand, recently, a step zooming method has been introduced in the field of the silver-halide camera which incorporates a zoom lens barrel, aiming at a miniaturization and cost reduction of the camera. As an example of the step zooming method, a technique set forth in (1) Japanese Patent Tokkaihei 08-94907 will be described in the following.
According to the step zooming method, a zooming region between the longest focal length and the shortest focal length is divided into a predetermined number of steps, namely, a stepping number of focal lengths. The step zooming method is described as follow, referring the zooming block diagram shown in FIG. 9. In FIG. 9, the horizontal axis indicates a variation of the focal length which is the shortest state at a position W, and successively becomes longer along M1, M2 and finally is the longest state at a position T. Accordingly, in this zoom lens, it is possible to change the focal length in four steps. The vertical axis indicates the moving amount of the front lens group and that of the rear lens group in the direction of the optical path, both of which are included in the zoom lens. Since the front lens group is helicoidally engaged with a rotating cam barrel driven by a motor, it moves in a straight direction, associating with the rotation of the lens frame. On the other hand, the rear lens group is driven by a cam formed on the cam barrel so that the rear lens group repeatedly moves between a focal position at which the shooting distance is ∞ (infinity) and a focal position at which the shooting distance is N (near distance) on the saw-tooth shaped locus.
For example, when the focal length is set at position W and a focusing operation is performed, only the front lens group moves between W and {circumflex over (1)}, corresponding to the shooting distance, while the rear lens group does not move. When single step zooming is performed in the telephoto-direction, both the front lens group and the rear lens group move to the position M1 after passing through the position {circumflex over (1)}. In the same way, when two-step zooming is performed in the telephoto-direction, both of them move to the position M1 after passing through the positions {circumflex over (1)}, M1 and {circumflex over (2)}. As mentioned above, since the focusing operation and the zooming operation are repeatedly performed by moving the front lens group and the rear lens group in the zoom lens, it becomes possible that a zooming mechanism and a focusing mechanism are integrated into the same mechanism. Accordingly, the structure of the camera is simplified by reducing the number of parts, and a small-sized zoom lens barrel can be attained.
Another example of a zoom lens barrel employing the step zoom method is set forth in (2) Japanese Patent Tokkaihei 6-313831, disclosing a configuration of the zoom lens barrel in which the switching operation of the focal length and the operation of focus adjustment are achieved by means of the same driving means including a motor, a cam, etc. and are selected in a stepping mode.
Although, in a conventional zoom lens barrel, a switching operation of the focal length, namely a zooming operation, and an operation of focus adjustment, namely a focusing operation, are performed by means of separate mechanisms, respectively, the disclosed configuration of the zoom lens barrel makes it possible to perform a zooming operation and a focusing operation with the same mechanism. Therefore, a very small-sized zoom lens barrel is attained.
The abovementioned zoom lens barrel employs a so-called step zoom method in which the length of the extended stroke between the position of the longest focal length and the position of the shortest focal length is divided into a predetermined number of variable power regions, in other words, divided into stepped number of focal lengths. This step zoom method will be described, referring FIG. 1. In FIG. 1, the vertical axis indicates the moving amount of the front lens group and that of the rear lens group in the direction of the optical path, both of which are included in the shooting lens, and further the horizontal axis indicates variations of the focal length. The focal length is the shortest state at a position W, and successively becomes longer along M1, M2-M6, and finally is the longest state at a position T. Accordingly, it is possible to change the focal length in eight steps.
The zoom lens barrel can be further shrunk from the position W to a storing position, so that the zoom lens barrel is housed in the storing region of the camera main body to miniaturize the size of the camera. Incidentally, since the miniaturization of the camera overrides other factors in the storing region, ranges from all subjects are apt to be optically out of focus. Therefore, the storing region is regarded as an impractical region for capturing images.
In addition, the zoom lens barrel can be further extended from the position T (∞) to an extended region, which can be utilized to prevent a moving restriction of the shooting lens due to a change of the stop position, which is caused by the focus adjustment of the lens at a reference length (position ∞, in this example) in the long focal length, and by variations of the stop position caused by the motor inertia when driving the shooting lens, and by variations of the mechanism rising from design conditions.
The front lens group moves linearly by means of a helicoidal engagement with a rotating cam barrel linked to the motor, while the rear lens group is driven by a cam formed on the cam barrel. Thus, the focal length can be varied repeatedly between a focus adjustment position at an infinity (∞) and that at a near range (N).
For example, when the focal length is set at position W, the shooting lens is focused at infinity (∞). When performing focus adjustment, the front lens group and the rear lens group move between W and M1, corresponding to the image shooting range. Accordingly, the focal length varies in accordance with the focus adjustment, and reaches the focal length of M1 at the near range (N). Performing single step zooming operation toward telephoto-direction, the front lens group and the rear lens group move to the position M1, and the shooting lens is focused at the near range (N). When the focus adjustment is conducted at this position, the front lens group and the rear lens group move between M1 and M2, corresponding to the image shooting range. Accordingly, the focal length varies in accordance with the focus adjustment, and reaches the focal length of M2 at the infinity (∞).
When the image capturing is finished after the focus adjustment, the zoom lens returns to one of the initial positions, W-T. In other words, the zoom lens returns to the position of the shortest focal length in each variable power region.
Since the focusing operation and the zooming operation are performed successively and repeatedly in the abovementioned configuration of the zoom lens barrel, the front lens group and the rear lens group can be driven by a single mechanism, being utilized for both focusing and zooming operations. This configuration decreases the number of necessary parts, resulting in a simpler camera structure, and attains a small-sized zoom lens barrel at low cost.
It is difficult, however, to realize a step zoom lens barrel having many steps as well as aiming at a small-sized zoom lens barrel as mentioned above, since the diameter and the rotatable angle of the cam barrel for moving the rear lens group are limited. Therefore, the number of selectable focal lengths for the user is also quite limited.